1. Field of the Invention
The invention relates generally to communication systems and, more particularly, to multiple access protocols for supporting users having different priority classes.
2. Discussion of Related Art
In today's information age, there is an increasing need for high speed communications that provides guaranteed quality of service (QoS) for an ever-increasing number of communications consumers. To that end, communications networks and technologies are evolving to meet current and future demands. Specifically, new networks are being deployed which reach a larger number of end users, and protocols are being developed to utilize the added bandwidth of these networks efficiently.
One technology that has been widely employed and will remain important in the foreseeable future is the shared-medium network. A shared medium network is one in which a single communications channel (the shared channel) is shared by a number of end users such that uncoordinated transmissions from different end users may interfere with one another. In modern broadband communications networks, the shared communications channel is typically one of a number of frequency bands carried over a shared physical medium, such as a hybrid fiber-optic/coaxial cable (HFC) network or by electromagnetic waves in free space. Since communications networks typically have a limited number of communications channels, the shared medium network allows many end users to gain access to the network over a single communications channel, thereby allowing the remaining communications channels to be used for other purposes. However, the shared medium network is only feasible when each end user only transmits data intermittently, allowing other end users to transmit during periods of silence.
In the shared medium network, each end user interfaces to the shared channel by means of an Access Interface Unit (AIU) which allows the end user to transmit and receive information via the shared channel. A single AIU may support one or a number of end users. Each end user wishing to utilize the shared channel participates in a Medium Access Control (MAC) protocol which provides a set of rules and procedures for accessing the shared channel. For convenience, each participant in the MAC protocol is referred to as a MAC User.
One type of shared medium network utilizes a single headend unit for coordinating access by the MAC Users to the shared channel. The headend unit is typically situated at a common receiving end of the shared channel and is able to transmit messages to all MAC Users that share the channel. The headend unit coordinates access to the shared channel by sending control messages to the MAC Users which enable one or more MAC Users to transmit. MAC Users only transmit when enabled to do so by the headend unit.
FIG. 1 shows an exemplary shared medium network 100 as is known in the art. As illustrated in FIG. 1, a headend unit 110 is coupled to a plurality of AIUs 120.sub.a through 120.sub.n (collectively referred to as AIUs 120) via a shared channel 130. In the preferred embodiment, the shared channel 130 is one of a number of communications channels carried by a shared physical medium such as a hybrid fiber-optic/coaxial cable (HFC) or wireless network. In other embodiments, the shared physical medium may be coaxial cable, fiber-optic cable, twisted pair wires, and so on, and may also include air, atmosphere, or space for wireless and satellite communication. The headend unit 110 is also coupled to a communications network 140, which may include networks such as the Internet, on-line services, telephone and cable networks, and other communication systems.
Continuing to refer to FIG. 1, in the preferred embodiment, the shared physical medium, such as an HFC or wireless network, has or supports a plurality of communications channels. For ease of reference, the communications channels in which a headend unit, such as the headend unit 110, transmits information, signals, or other data to an AIU, such as AIU 120.sub.n, are referred to as downstream channels. Also for ease of reference, the communications channels in which an AIU, such as AIU 120.sub.n, transmits information, signals, or other data to a headend unit, such as headend unit 110, are referred to as upstream channels. These various upstream and downstream channels may, of course, be the same physical channel, for example, through time-division multiplexing and duplexing. They may also be separate physical channels, for example, through frequency-division multiplexing and duplexing. These various channels may also be logically divided in other ways, in addition to upstream and downstream directions. In the preferred embodiment, the communications medium is an HFC network, with downstream channels in the frequency spectrum (band) typically 50-750 MHz (and up to 1 GHz), and with upstream channels in the frequency spectrum typically 5-42 MHz.
In a simple model of an exemplary HFC network, the headend unit uses a single downstream channel to send information to a group of MAC Users, and a single upstream channel is used by all (or a number) of the group of MAC Users to send information to the headend unit. Since the headend unit is the only device which transmits on the downstream channel, the downstream channel is not a "shared channel" as that term is applied to the present invention. However, since multiple MAC Users transmit on the upstream channel, the upstream channel is a shared channel, and the MAC protocol must provide for orderly access to the channel so as to maximize the data throughput over the channel.
A number of different MAC protocols have been developed for use over a shared medium network. These protocols can generally be categorized as contention-free protocols, which avoid collisions on the shared channel by means of various scheduling methods, and contention-based protocols, which do not avoid collisions but instead resolve any collisions that do occur on the shared channel. Contention-free protocols, such as time-division multiple access (TDMA) and round-robin polling, are typically less efficient than contention-based protocols under light loads (i.e., many idle MAC Users) because the contention-free protocols generally allocate some amount of bandwidth to each MAC User whether or not the MAC User has information to send. On the other hand, contention-based protocols only allocate bandwidth to those MAC Users that have information to send, although some amount of bandwidth is wasted whenever collision resolution is required. Under heavy loads (i.e., many active MAC Users), there are likely to be many collisions. Thus, the effectiveness of a MAC protocol is generally dependent on the number of MAC Users as well as the amount of information they have to send.
One type of MAC protocol utilizes a reservation system in which each MAC User that wants to transmit data on the shared channel is required to make a reservation with the headend unit. Each MAC User that has data to transmit but has not already made a reservation waits for contention opportunities provided by the headend unit. Each contention opportunity is provided to a selected group of MAC Users, and allows each of the MAC Users in the specified group to contend for a reservation at a specific time provided it has data to send. For convenience, the term "data" is used generically to mean a collection of bits of information that may carry various forms of multimedia signals (e.g., voice, video, etc.).
In a typical contention-based reservation system such as the system described immediately above, the MAC Users contend for the shared channel as peers. In other words, when a number of MAC Users are given an opportunity to contend, any of the number of MAC Users having data to transmit may contend. When the MAC Users are distinguishable by different QoS requirements, low-priority MAC Users can force collisions with high-priority MAC Users, and collision resolution may allow low-priority MAC Users to transmit before high-priority MAC Users. When low-priority MAC Users prevail over high-priority MAC Users, it becomes more difficult for the system to meet the QoS requirements of the high-priority MAC Users. Therefore, a need remains for a system, device, and method for sharing contention mini-slots among multiple priority classes.